Lou,
The condition you are describing sounds like an issue from a time long ago
when selective soldering was common. The process would create a demarcation
line at the pad junctions that would etch out during the etching process.
Many time the physical size of the demarcation would be so small that you
would actually create a perfect line dish down. Meaning up to 90% of the
copper thickness would be removed while leaving a very small amount of base
copper to retain the conductivity of the trace. This would allow the bare
board to pass test but could result in subsequent failure after additional
thermal stresses.
I'm uncertain as to what you are actually seeing. ( perhaps you could
provide a picture of the open). In any case, failure due to an open at the
junction of a pad and trace would typically be a bare board issue, unless
you have reworked the areas and created a lifted land condition.
Bye chance are the SMT pads gold plated. If so, contact me off line. I might
have possible explanation surrounding selective electrolytic gold plating.
Just a thought.
Ed Cosper
ABC
----- Original Message -----
From: Lou Hart <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, December 15, 1999 9:24 AM
Subject: [TN] Open traces on printed circuit boards
Technetters, I'd like to have a clearer, more precise question to ask, but
maybe you can help with any comments from experience.
Our fairly small assembly shop has been troubled with open traces, found at
final test, on the circuit boards of some pretty expensive assemblies. The
bare boards come from our sister division of the same company. They are
sizeable multilayers with traces 4 or 5 mils in width. The opens appear
most commonly near the surface mount pads of a cluster of SSOPs (0.65 mm
pitch).
Test here had believed that the bare board test might not be catching the
opens. The PC shop suggested the opens were coming from overstress during
assembly testing. But the appearance of these opens did not resemble that
of traces that had been overstressed in test.
This week one of the test techs found an assembly that worked OK, then an
open appeared. This behavior seemed to support the thesis, proposed by the
PC shop, that the traces could have been greatly reduced during fabrication,
but were conductive enough to pass bare board test. (Bare board test
delivers 20 ma for the continuity test.) In the operating assembly, the
trace could carry as much as 200 ma, and for a longer time than that
consumed by the bare board test which could blow the tiny trace.
Many thanks for any comments or ideas. Lou Hart Compunetix 412-858-6184
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